Electrochemical devices or systems of the type referred to herein include one or more of the metal-halogen battery systems, such as a zinc-chloride battery system. These metal-halogen battery systems generally are comprised of three basic components, namely an electrode stack section, an electrolyte circulation subsystem, and a store subsystem. The electrode stack section typically includes a plurality of cells connected together electrically in various series and parallel combinations to achieve a desired operating voltage and current at the battery terminals over a charge/discharge battery cycle. Each cell is comprised of a positive and negative electrode which are both in contact with an aqueous metal-halide electrolyte. The electrolyte circulation subsystem operates to circulate the metal-halide electrolyte from a reservoir through each of the cells in the electrode stack in order to replenish the metal and halogen electrolyte ionic components as they are oxidized or reduced in the cells during the battery cycle. In a closed, self-contained metal halogen battery system, the storage subsystem is used to contain the halogen gas or liquid which is liberated from the cells during the charging of the battery system for subsequent return to the cells during the discharging of the battery system. In the zinc-chloride battery system, chlorine gas is liberated from the positive electrodes of the cells and stored in the form of chlorine hydrate. Chlorine hydrate is a solid which is formed by the store subsystem in a process analogous to the process of freezing water where chlorine is included in the ice crystal.
With reference to the general operation of a zinc-chloride battery system, an electrolyte pump operates to circulate the aqueous zinc-chloride electrolyte from a reservoir to each of the positive or "chlorine" electrodes in the electrode stack. These chlorine electrodes are typically made of porous graphite, and the electrolyte passes through the pores of the chlorine electrodes into a space between the chlorine electrodes and the opposing negative or "zinc" electrodes. The electrolyte then flows up between the opposing electrodes or otherwise out of the cells in the electrode stack and back to the electrolyte reservoir or sump.
During the charging of the zinc-chloride battery system, zinc metal is deposited on the zinc electrode substrates and chlorine gas is liberated or generated at the chlorine electrode. The chlorine gas is collected in a suitable conduit, and then mixed with a chilled liquid to form chlorine hydrate. A gas pump is typically employed to draw the chlorine gas from the electrode stack and mix it with the chilled liquid, (i.e., generally either zinc-chloride electrolyte or water). The chlorine hydrate is then deposited in a store container until the battery system is to be discharged.
During the discharging of the zinc-chloride battery system, the chlorine hydrate is decomposed by permitting the store temperature to increase, such as by circulating a warm liquid through the store container. The chlorine gas thereby recovered is returned to the electrode stack via the electrolyte circulation subsystem, where it is reduced at the chlorine electrodes. Simultaneously, the zinc metal is dissolved off of the zinc electrode substrates, and power is available at the battery terminals.
Over the course of the zinc-chloride battery charge/discharge cycle, the concentration of the electrolyte varies as a result of the electochemical reactions occurring at the electrodes in the cells of the electrode stack. At the beginning of charge, the concentration of zinc-chloride in the aqueous electrolyte may typically be 2.0 molar. As the charging portion of the cycle progresses, the electrolyte concentration will gradually decrease with the depletion of zinc and chloride ions from the electrolyte. When the battery system is fully charged, the electrolyte concentration will typically be reduced to 0.5 molar. Then, as the battery system is discharged, the electrolyte concentration will gradually swing upwardly and return to the original 2.0 molar concentration when the battery system is completely or fully discharged.
Further discussion of the structure and operation of zinc-chloride battery systems may be found in the following commonly assigned patents: Symons U.S. Pat. No. 3,713,888 entitled "Process For Electrical Energy Using Solid Halogen Hydrates"; Symons U.S. Pat. No. 3,809,578 entitled "Process For Forming And Storing Halogen Hydrate In A Battery"; Carr et al. U.S. Pat. No. 3,909,298 entitled "Batteries Comprising Vented Electrodes And Method of Using Same"; Carr U.S. Pat. No. 4,100,332 entitled "Comb Type Bipolar Electrode Elements And Battery Stack Thereof"; Whittlesey et al. U.S. Pat. No. 4,518,664 entitled "Comb-Type Bipolar Stack". Such systems are also described in published reports prepared by the assignee herein, such as "Development of the Zinc-Chloride Battery for Utility Applications", Interim Report EM-1417, May 1980, and "Development of the Zinc-Chloride Battery for Utility Applications", Interim Report EM-1051, April 1979, both prepared for the Electric Power Research Institute, Palo Alto, Calif. The specific teachings of the aforementioned cited references are incorporated herein by reference.
The present invention is directed to an improved comb-type bipolar electrode stack construction, and is particularly directed to an improved intermediate bus bar construction within the comb-type bipolar stack constructions, for electrochemical systems having circulating electrolyte, which are particularly advantageous in zinc-chloride battery systems. Prior comb-type bipolar stacks for zinc-chloride battery systems have included intermediate bus bars for receiving the chlorine and zinc electrodes which are generally supported only at the periphery. Such structures are disclosed in the commonly assigned Carr et al. U.S. Pat. No. 4,100,332 and Whittlesey et al. U.S. Pat. No. 4,518,664, which have been previously incorporated by reference. This latter patent illustrates battery stack configurations which are based upon either an "open" top submodule design or a "closed" top submodule design. In the closed top submodule design, the submodules themselves are sealed, and therefore they will be directly subject to the various pressure levels at which the battery stacks operate during a charge/discharge cycle.
Since the stack area of a zinc-chloride battery may operate under a vacuum during one portion of a charge/discharge cycle and under pressure during another portion of this cycle, it should be appreciated that the bus bars in a closed top submodule design will be subject to the stress induced by these pressure levels (e.g. +/-10 psig). This pressure-induced stress could cause the intermediate bus bars to slightly bow or deform, and eventually such stress could lead to structural cracks along the edges of the bus bars where the plastic frame provides support for the bus bar. Since it is possible for a structural crack in the bus bar to compromise the electrolyic separation between the cells, and thereby adversely affect the efficiency of these cells, it would be advantageous to find an economic way to provide further support for the intermediate bus bars, particularly in connection to the closed top submodule design. Additionally, even slight elastic deformation or bowing along the length of the bus can cause deformation of the electrodes supported by the bus which may result in alteration of the inter-electrode gap and alteration of the cell geometry, thereby adversely affecting the performance of the cell.
Furthermore, stressful conditions are also created for the zinc-chloride battery bus bars when the porous graphite electrodes in the cell assemblies undergo an "activation" process to decrease the oxidation and reduction chlorine overvoltages of these electrodes. A detailed discussion of typical activation processes may be found in the following commonly assigned patents: Hart U.S. Pat. No. 4,120,774 entitled "Reduction of Electrode Overvoltage"; Laetham et al. U.S. Pat. No. 4,273,839 entitled "Activating Carbonaceous Electrodes". The specific teachings of these references are hereby incorporated by reference. Due to the nature of these activation processs, cell structures containing porous graphite electrodes are placed in a substantially more stressful environment than encountered during normal battery operation.
Accordingly, it is a principal objective of the present invention to provide an improved bus bar design for an electrode cell design which is subject to pressure and/or vacuum during a charge/discharge cycle.
It is a more specific object of the present invention to provide an intermediate bus bar structure which is reinforced for increased mechanical strength.
It is an additional object of the present invention to provide a reinforced intermediate bus bar structure for a zinc-chloride battery system which may readily be employed in a comb-type cell element providing the basic building block for constructing closed top submodule electrode stacks.
It is another object of the present invention to provide a reinforced intermediate bus bar structure which can more easily withstand an activation process.
It is an objective of the present invention to provide a reinforced intermediate bus bar frame design for a bus bar which physically supports electrodes placed therein, reducing deformation of the bus bar and to maintain a desired inter-electrode gap.
It is another objective of the present invention to provide a reinforced bus bar frame design which may be injection molded to enhance manufacturability and assembly of the battery stack.
To achieve the foregoing objects, the present invention generally comprises an intermediate bus bar structure and a plastic frame which is reinforced with vertical straps. In a preferred embodiment of the invention, grooves are provided in the bus bars to hold the straps flush to the faces of the bus bars, and the straps are preferably molded into these grooves during the molding of the frame around the bus bar. The straps which are provided at one or more locations along the length of the bus member improve the structural integrity of the bus member.
Additional advantages and features for the present invention will become apparent from a reading of the detailed description of the preferred embodiments which make reference to the following set of drawings.